1. Technical Field
The present disclosure relates to an oscillation device that includes a PLL for generating an oscillation output corresponding to a frequency setting value, based on a clock signal generated during adjustment of a temperature of an atmosphere where a crystal unit is placed.
2. Description of the Related Art
In the case where a crystal controlled oscillator is incorporated into an oscillation device requiring extremely high frequency stability, an oven controlled crystal oscillator (OCXO) is typically used. Temperature control of the OCXO is performed with a configuration that employs a thermistor as a temperature sensor and discrete parts such as an operational amplifier, a resistor, and a capacitor. However, a temperature control within, for example, ±20 m° C. cannot be performed due to variation and aging of individual analog parts.
However, for example, a base station and a relay station require use of a clock signal with extremely high stability at low price. Consequently, the conventional OCXO is expected to have difficulty in dealing with some situations. Especially, even when the temperature control is performed like in the OCXO, for example, in the case where change in temperature of the atmosphere where the crystal controlled oscillator is placed occurs based on rapid change in external temperature or a trouble occurs in the temperature control, this may cause decrease in stability of frequency.
FIG. 2 and FIG. 3 of Japanese Unexamined Patent Application No. 2001-292030 disclose that two pairs of electrodes are disposed on the common crystal element to constitute two crystal units (crystal resonators). The paragraph (0018) discloses that a frequency difference appears between the two crystal units corresponding to a temperature change and therefore measuring this frequency difference is equivalent to measuring the temperature. The ROM stores the relationship between the frequency difference Δf and the compensation amount of frequency. The compensation amount of frequency is read out based on Δf.
However, this method relates to a temperature compensated crystal oscillator (TCXO) that compensates an oscillation frequency based on temperature detection, but does not relate to the OCXO.
As disclosed in the paragraph (0019), it is necessary to adjust the crystal units such that the desired output frequency f0 and the respective frequencies f1 and f2 of the two crystal units satisfy the relationship of f0≈f1≈f2. Therefore, a problem arises in that this complicates the production process of the crystal unit and does not provide high yield. Additionally, the clock that is the frequency signal from each crystal unit is counted for a certain period of time to obtain the difference (f1−f2). The detection time is directly affected by the detection accuracy. Therefore, it is difficult to perform temperature compensation with high accuracy.
The present disclosure is made under such a background, and provides an oscillation device that obtains an oscillation output with high frequency stability regarding an oscillation device that includes a crystal oscillator (OCXO) that detects a temperature of an atmosphere where a crystal unit is placed and controls a heating unit based on a result of the detection to have a constant temperature of the atmosphere.